A belt-type continuously variable transmission is known as a variable transmission installed in an automobile or the like. The belt-type continuously variable transmission includes a primary pulley, to which drive force of the internal combustion engine is transmitted, a secondary pulley, which is coupled to the wheels, and a belt, which runs around the pulleys. The running radius of the belt differs for each pulley so that the gear ratio varies continuously and steplessly.
Such a belt-type continuously variable transmission increases and decreases the oil pressure of the oil pressure chamber in each pulley to vary the width of a groove extending along the pulley. This varies the running radius of the belt on the pulley and controls the gear ratio.
For example, to decrease the gear ratio, transmission control oil pressure, which is the oil pressure of the oil pressure chamber arranged in the primary pulley, is increased to narrow the groove width of the primary pulley. This increases the running radius of the belt on the primary pulley. At the same time, narrowing control oil pressure, which is the oil pressure of the oil pressure chamber arranged in the secondary pulley, is decreased to widen the groove width of the secondary pulley. This decreases the running radius of the belt on the secondary pulley. Consequently, the gear ratio is varied while suppressing slipping of the belt on the pulleys.
To increase the gear ratio, the transmission control oil pressure is decreased to widen the groove width of the primary pulley and decrease the running radius of the belt on the primary pulley. Further, the narrowing control oil pressure is increased to narrow the groove width of the secondary pulley and increase the running radius of the belt on the secondary pulley.
A controller that controls such a belt-type vehicle continuously variable transmission sets a target gear ratio in accordance with the depression amount of the accelerator pedal, the vehicle speed, the engine speed, and the like. The controller feedforward controls the transmission control oil pressure based on the target gear ratio and the narrowing control oil pressure. During the feedforward control, the controller also calculates the actual gear ratio based on the rotation speed of the primary pulley and the rotation speed of the secondary pulley to feedback control the transmission control oil pressure based on the deviation of the calculated actual gear ratio and the target gear ratio.
When an electromagnetic pickup rotation speed sensor is used as a rotation speed sensor that detects the rotation speed of each pulley and the rotation speed is low, due to the sensor characteristics, the detection accuracy may become low or the rotation speed may not be detected. Thus, when the vehicle speed is in an extremely low vehicle speed range, such as when the vehicle is about to stop, the rotation speed of the secondary pulley may not be accurately detected and accurate feedback control may be hindered. As a result, the controller would not be able to have the gear ratio conform to the target gear ratio, and the gear ratio would not be accurately controlled. This may result in instable transmission control.
Patent document 1 and patent document 2 each describe a controller for a vehicle continuously variable transmission that releases operational oil from the primary pulley to lower the transmission control oil pressure when determined that the vehicle speed is less than a reference speed and within an extremely low vehicle speed range, in which a rotation speed sensor cannot accurately detect the rotation speed of the secondary pulley. Such a structure widens the primary pulley with the tension of the belt so that the groove width of the primary pulley, which has a variable range, is held to be maximal. Thus, the gear ratio may be held at the maximum gear ratio even in a situation in which feedback control cannot be executed.